Chlorine dioxide has been used in a variety of applications subsequent to its discovery in 1811 by Sir Humphry Davey. Applications that are commercially used today include the bleaching of wood pulp for paper making; bleaching of textile, flour, flower, cherry, tallow, and recycled paper; disinfection of industrial process water, drinking water, waste water, and food contact surfaces; use as a disinfectant and odor control agent in a variety of industries and applications; use for the destruction of pollutants such as cyanide and phenols; and use of its oxidation powers for industrial process purposes.
Chlorine dioxide use has become popular in many of these applications because of its strong oxidation power, quick and broad biocidal capability, and its ability to oxidize without the formation of unwanted chlorination by-products.
The one drawback to chlorine dioxide is that, because of its tendency to explode when compressed or stored in concentrations of over 10 percent, it must be produced on-site prior to use rather than produced at a central location and shipped to the point of use. This has lead to a variety of generation processes for its production. The two basic types of processes are differentiated by use of either sodium chlorite or sodium chlorate as the precursor chemical.
Sodium chlorite based processes are, in general, small, safe and very easy to operate. There are several patents disclosing these types processes. U.S. Pat. No. 4,250,144, issued to Ratigan, describes a process for the reaction of dissolved chlorine (hypochlorous acid) in water with sodium chlorite. U.S. Pat. No. 4,590,057, issued to Hicks, discloses a process where molecular chlorine is reacted directly with sodium chlorite prior to dilution in water to form chlorine dioxide. Though these are by far the two most common chlorite based methods, there are a profusion of other chlorite based methods that react sodium chlorite with various acids or oxidizers to form chlorine dioxide. Though they all are relatively simple to operate when compared to the chlorate based processes, the chlorine dioxide produced from chlorite is inherently from 4 to 5 times as expensive as that produced from a chlorate based process. Sodium chlorite is much more expensive to produce than sodium chlorate. Industries where chlorite based process are used are the water disinfecting, food, and industrial water treatment sectors where typical use capacities are under 2,000 lbs. per day.
Sodium chlorate based process are, in general, much larger and more complex to operate than a chlorite based process. U.S. Pat. No. 3,920,801 issued to Grotheer describes an electrolytic cell process where sodium chlorate is reacted through a cascade system with hydrochloric acid and heat to produce chlorine dioxide and chlorine. U.S. Pat. No. 4,081,520 issued to Swindells et al, describe a process where sodium chlorate is reacted with a mixture of liquid sulfuric acid and methanol to form chlorine dioxide. U.S. Pat. No. 3,755,068, issued to Rapson, describes a process where sodium chlorate is reacted with a mixture of sodium chloride and acid to form chlorine dioxide and chlorine. U.S. Pat. No. 2,373,870, issued to Holst, discloses a process where sulfuric acid is reacted with sodium chlorate and sulfur dioxide to form chlorine dioxide.
Sodium chlorate reactors are extremely capital intensive, with generator costs ranging from $2,500,000.00 to $10,000,000.00 compared with $3,500.00 to $100,000.00 for chlorite based processes. Chlorate units also require constant operator supervision where chlorite based process operate for months on end without operator supervision. The chlorite generator because of its small reaction column, vacuum chemical feed, and simplicity is safer, and produces a more pure chlorine dioxide stream than the chlorate based unit.
Because of the cost and operational differences of the chlorite and chlorate based processes, there has been a tremendous drive to "downscale" chlorate processes to chlorite generation systems. Though there have been several patents granted on these systems, none has been commercially successful. An example of these would be U.S. Pat. No. 4,451,444, issued to Santillie et al. This is because of the inherent nature of the reactants used require longer reaction times than the chlorite based processes. Retention time in a chlorite based process varies from minimal (&lt;0.5 seconds) in the Hicks et al process to a maximum of 15 minutes as described in the acid chlorite type processes. These reactions are carried out at room temperature. In contrast the chlorate based processes allow for 2 to 4 hours of retention time, or use multiple pass reactors such as described by Day et al, U.S. Pat. No. 2,484,402, and operate at temperatures commonly exceeding 90.degree. C.